in 1982, I am part of the last generation that experienced the
manual focus on film before the generalization of AF. However,
even today, in the achievement context of very specific photographs,
manual focus is still a feasible option and accessible at all
times, even on the latest digital compact all available on the
facilitate the focus, one has to engineer originally from the
School of Fine Arts named Lucien Dodin, the invention of the
stigmometer in 1943 that he preferred
to call "cut rangefinder field" or "rangefinder
Dodin". The stigmometer belongs to the large family of rangefinders
and specifically telemetry coincidence lines. Incorporated in
standard focusing screen many analog cameras before the autofocus
advent, it allowed a simple and precise principle to focusing
on any topic. It curiously recalls the Foucault knife principle.
This spot divides the image into two. If the point of sharpness
was not confused with the film plane, the image appeared clearly
divided. To develop the image was to act on the lens until the
image appears uniform, without division.
has eliminated the dreaded stigmometer on all SLR cameras since
the early 90's. This is an electronic eye simplifying focus on
bringing much responsiveness to capture very fast scenes. Welcome
to the electronic telemetry with two passive systems. These systems
do not require signal transmission. This passive telemetry operates
only light reflected from the subject.
telemetry team image contrast measurement mainly compact cameras.
Less expensive, it is also less efficient than SLR based on the
phase contrast detection because this measure is the exploitation
of the image main sensor to perform the focus. Very random performance
of this autofocus type in difficult conditions are explained
by the nature of his considering procedure in the image sharpness
proportional to the contrast intensity. The image contrast measure
consists in comparing several successive images produced by the
of this measure makes the autofocus exceedingly swift little
agitated on a subject whose distance changes very frequently
between two successive measurements.
The DSLR autofocus
uses an electronic telemetry principle very different from that
of a compact. Phase contrast detection is the automated version
of the telemetry coincidence lines. Several sensors are coordinated
leading to a very similar operation in the Lucien Dodin stigmometer.
The autofocus principle is look the same image in two ways opposed
to the lens center and measure the symmetrical offset contrast
fringes of the two images as a lens to cut field.
to image contrast measurement, that of a DSLR does not search
point but the calculation only once from wherever symmetrical
shift between the two images and control the lens for cancel
the difference. This results in a much higher reactivity to that
of a compact as the unit will search any sharpness from multiple
images as a image contrast measure. The time savings and accuracy
are significant, all the more so with the simultaneous use of
very silent supersonic motor installed in the AF. Microprocessors
built by NIKON, CANON, SIGMA TAMRON.